I understand that it's not considered possible (or at least practical)
to test all package upgrades on all platforms. However, certain packages
have a large number of dependencies. Take glib2 for example. Likely as a
result of an upgrade from 2.28.8 to 2.30.2 on 29 Jan 2012, it now
doesn't build on DragonFly resulting in 1849 breakages (1760 directly).

Would it be a good idea to identify packages like this one that have a
large number of direct dependencies (say > 100) and require that any
version upgrade be tested on a standard set of platforms before getting
committed?

Sounds reasonable to me, but how do you perform this testing? Would
developers have access to build boxes to try their packages before
submitting them? Or would this be automated somehow?